In the conventional field of optics, the focal spot size has been restricted due to the diffraction limit of light. In recent years, however, near-field light that can exceed the limit has attracted attention, and investigations using the near-field light have been vigorously conducted in various fields, for example, using a scanning near field optical microscope (SNOM) capable of observing an object of nanometer size. Among the applications of the near-field light, surface plasmon resonance has particularly attracted attention, which makes it possible to obtain electric field intensity of tens of times of that of incident light. Here, the surface plasmon resonance means a phenomenon that plasma oscillation of free electrons, which are generated locally in a metal surface layer when external electromagnetic waves are applied thereto, comes to resonate with the applied electromagnetic waves.
Japanese Patent Laying-Open No. 1-138443 discloses a device for causing the surface plasmon resonance. FIG. 6 is a schematic cross sectional view of a basic device for causing the surface plasmon resonance. This device includes a light source 101, a light converging lens 102 for converging light emitted from the light source, a triangular prism 103 formed of a transparent dielectric, a thin metal film 104 formed on a surface of the triangular prism, and a photodetector 105 for detecting light reflected by the thin metal film.
P-polarized light emitted from light source 101 is transformed by light conversing lens 102 to convergent light, which is transmitted through triangular prism 103 and focused on thin metal film 104 at an incident angle θ. Incidentally, “p-polarized light” means linearly polarized light in which the electric vector of the light incident on a substance surface has a vibration direction that lies in a plane including the traveling direction of the light and a line normal to the substance surface. A part of the light focused on thin metal film 104 satisfies the resonance condition and comes to resonate with a surface plasmon to cause an enhanced evanescent field 106 on the free surface side of thin metal film 104. The remaining part of the light is reflected and then is detected by photodetector 105.
A graph shown in FIG. 7 is obtained by changing the incident angle θ of light on thin metal film 104. In this graph, a horizontal axis represents the incident angle θ of light, and a vertical axis represents reflectance (%). In FIG. 7, the intensity of light received by photodetector 105 becomes a minimum at a specific incident angle θs, indicating that a part of the convergent light resonates with a surface plasmon at that incident angle.
Japanese Patent Laying-Open No. 5-240787 discloses an application of the above-described device for exciting surface plasmons to a microscope. FIG. 8 is a schematic cross sectional view of a basic microscope utilizing surface plasmons. This figure shows a light source 201, a beam expander (lenses 202, 203) for expanding parallel light emitted from the light source, a light converging lens 204 for transforming the parallel light expanded by the beam expander into convergent light, a prism 205 for coupling the light, a thin metal film 206 formed on a surface of prism 205, a specimen 208 separated from the thin metal film with a gap filled by emulsion oil 207, a photodetector 209 for detecting the light reflected by thin metal film 206, and an X-Y pulse stage 210 for moving specimen 208 intermittently.
Parallel light emitted from light source 201 is expanded by beam expander 202, 203, and transformed by light converging lens 204 to convergent light which is then transmitted through prism 205 and focused on thin metal film 206. Of the focused light, a light part having a specific incident angle excites a surface plasmon. The incident angle depends on the thicknesses and refractive indices of thin metal film 206, emulsion oil 207 and specimen 208.
The light reflected by thin metal film 206, without contributing to excitation of the surface plasmon, is measured by photodetector 209. Photodetector 209 detects coordinates where intensity of the reflected light is reduced due to excitation of the surface plasmon, and then a surface plasmon excitation angle can be obtained from the coordinates, thereby making it possible to determine change in refractive index of specimen 208. Further, X-Y pulse stage 210 is used to scan specimen 208 so as to obtain two-dimensional distribution of the refractive index of the specimen.
With the device shown in FIG. 6 or FIG. 8, however, the area where the surface plasmon is excited depends on the spot size of the focused light. For example, in FIG. 6, if light source 101 has a wavelength of 650 nm and light converging lens 102 has an NA (numerical aperture) of 0.6, the light beam can be narrowed only to a diameter of about 1 μm. This means that the microscope of FIG. 8 can obtain a resolution only on the order of 1 μm. That is, the resolution limit of a microscope is determined by the diffraction limit of light emitted from the light source.
On the other hand, it is possible to reduce the spot size to a certain degree by decreasing the wavelength of the source light and increasing the NA of the light converging lens. However, it is extremely difficult to obtain a small light spot of an nm order size. Thus, it appears that the resolution of the conventional microscope utilizing surface plasmons has already reached a critical limit.